News Release Archive:

News Release 845 of 1025

November 2, 1995 12:00 AM (EST)

News Release Number: STScI-1995-44

Embryonic Stars Emerge from Interstellar "Eggs"

Background information useful for exploring this news release:

M16 - Star Birth in the Nest of the Eagle

Stars are born from the gas of interstellar space. When they eventually burnout and die, they bequeath their legacy back to the interstellar medium from which they formed. The signposts marking this ongoing cycle of birth, death, and renewal would be easily visible to any casual observer who had a bird's-eye view of our pinwheel-shaped galaxy. Spread across our galaxy such an observer would see majestic spiral arms, highlighted by bright young stars and the glowing clouds of gas that those stars illuminate.

On a clear, dark summer night earth-based observers can see these glowing clouds, called nebulae, scattered along the track of the Milky Way. Many can be found by looking in the direction of the great star clouds in the summer constellation, Sagittarius.

One of the most unique star-birth regions is the Eagle Nebula, (also called M16 because it is in the Messier Catalog of "fuzzy" permanent objects in the sky, that was compiled more than 200 years ago by French astronomer Charles Messier) it is visible in binoculars near the border between the constellations of Sagittarius and Serpens. The nebula is actually a bowl-shaped blister on the side of a dense cloud of cold interstellar gas.

Most of this cloud is so dense and cool that its hydrogen atoms are bound as molecules. This "molecular hydrogen" is the raw material for building new stars. The cloud contains microscopic dust particles of carbon (in the form of graphite), silicates and other compounds similar to those found in terrestrial and lunar rocks. Though this trace dust accounts for only a fraction of the nebula's mass, it's enough dust to absorb visible light — cloaking some of the visual details of star birth.

A cluster of about 100 newborn stars glitters inside the open "bowl" of the nebula. A few of these stars are much more massive than our Sun is, and so are tremendously hotter and brighter than the Sun. The brightest of these stars may be 100,000 times brighter than the Sun and have temperatures of nearly 90,000 degrees Fahrenheit (50,000 degrees Kelvin).

These young stars emit intense ultraviolet radiation which is so energetic it heats the surrounding gas, causing it to glow like the gas inside a fluorescent light bulb. When this ultraviolet light hits the bowl-shaped surface of the molecular cloud, it heats that gas, causing it to "evaporate" and stream away from the surface. If one could watch the process for more than a million years, they would see the bowl grow increasingly larger as the radiation from the stars eats deeper into the molecular cloud.

Unlike other stellar nebula which we see face-on — like the great Orion Nebula — M16 presents astronomers with a unique side view of the structure of a typical star-birth region: the cluster of hot, young stars in the center of the cavity, the evaporating surface of the cloud, and finally the great cold mass of the cloud itself.

The Eagle Nebula's name comes from its symmetrical appearance which is reminiscent of a bird of prey with outstretched wings and talons bared. The Eagle's "talons" are actually a series of dense columns of gas that protrude into the interior of the nebula. These columns form as a result of the same process that causes the bowl to grow. Because the columns are denser than their surroundings, they are not evaporating as rapidly as the surrounding gas, and so remain. The process is analogous to the formation of towering buttes and spires in the deserts of the American Southwest. These geological features formed when wind and rain eroded away softer ground, but places where the rock was harder resisted erosion and were left behind.

Inside these interstellar columns, the gas density can get so high that gravity takes over and causes the gas to start collapsing into ever-smaller clumps. As more and more gas falls onto these growing clumps they get further compressed by their own weight, until finally they trigger nuclear fusion reactions in their cores, and "turn on" as stars.

However, in M16 this process may not get a chance to go on to completion. If a forming star and the gas cloud that surrounds it are "uncovered" by photoevaporation before the star finishes growing, the mass of the young star may be "frozen." The star can't grow any more simply because the cloud from which it was drawing material is gone. In M16 Hubble Space Telescope's high resolution seems to have caught about 50 stars in this situation.

These are called EGGs "evaporating gaseous globules." The acronym is appropriate because these EGGs are objects within which stars are being born and are now emerging.

M16 is where the action is today, but it won't remain so forever. Within another few million years, star formation will have exhausted or dispersed the available raw material, and the massive stars that illuminate the Eagle will have lived out their short lives and died in spectacular supernova explosions. But even though the "birth cloud" nebula will be gone, most of the stars that formed there will remain. The offspring of the Eagle will "take wing" among the rest of the hundreds of billions of stars that make up our galaxy.

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